The present invention relates to a light emitting device having nano structures for light extraction and a method for manufacturing the same.
In general, a light emitting diode is one of light sources with a single wavelength, which has various applications such as light sources for cars, electronic display boards, lighting devices, and backlight units for display devices.
Light generated in a light emitting diode is mostly confined inside the diode due to total internal reflection with a critical angle at an interface between two media such as a semiconductor and air.
FIG. 1 is a conceptual diagram illustrating a light path between two media having different refraction indices. When light travels from a first medium having a refraction index of ‘n1’ to a second medium having a refraction index of ‘n2’, the light travels per the Snell's law that is expressed by Equation 1. The light incident on the second medium from the first medium with an angle less than the critical angle passes through an interface in-between and light incident at an angle larger than the critical angle undergoes a total internal reflection.ni*sin θ1=n2*sin θ2  (1)
Where, θ1 is an incident angle and θ2 is a refraction angle.
FIG. 2 is a schematic sectional view showing a light path in a typical light emitting diode. In the structure of a light emitting diode in which an n-semiconductor layer (11), an active layer (12) and a p-semiconductor layer (13) are laminated in sequence on a substrate (10), light (a, b, c) traveling to the outside of the device at an angle less than a critical angle among light emitted from the active layer (12) passes through an interface.
However, light (d) traveling toward the outside of the device at an angle (θ3) larger than the critical angle is totally reflected and confined within the device.
Therefore, as the amount of light confined within the device increases, the light output of the light emitting diode decreases and the characteristics thereof are degraded.
There are various methods for improving light extraction efficiency in a light emitting diode.
First, there is a method for increasing a probability of vertical incidence of light in a light emitting diode chip by changing the shape of the chip. It is known that it is theoretically optimum for a light emitting diode chip to have a hemispherical shape. However, disadvantages are that it is difficult to fabricate a light emitting diode chip having the hemispherical shape and fabrication costs thereof increase.
A second method is for encapsulating a light emitting diode with a hemispherical epoxy dome. However, it is difficult to fabricate such a light emitting diode.
A third is a technique for replacing a substrate capable of resorbing light emitted from a light emitting diode with a total-internal reflection substrate.
There is a method of fabricating a light emitting diode with a micro cavity or a resonant cavity structure. In this method, a very fine control and re-productivity are required for the thickness or the like of constitutional layers upon fabrication of the structure. In this case, efficient extraction of light from a semiconductor to air involves a difficulty in which an emission wavelength of a light emitting diode should precisely conform to a cavity mode.
Furthermore, the emission wavelength of a light emitting diode is changed to cause a rapid reduction in light output, if temperature or an operating current increases.
On the other hand, in order to improve light extraction efficiency of a light emitting diode, there have been recently proposed surface texturing techniques in which the surface of a light emitting diode chip for emitting light generated from the inside thereof to the outside is artificially roughened or formed with a regularly repeated pattern.
The surface texturing techniques are technique for improving light extraction efficiency in a light emitting diode chip. The techniques can further improve the light extraction efficiency by being employed in combination with other existing techniques such as a chip shape modification technique, an epoxy encapsulation technique, and a substrate modification technique.
The surface texturing techniques currently employ a method for providing a texture on a surface by forming a pattern using a mask or the like and performing wet or dry etching.
In these techniques, each layer in the structure of a light emitting diode has a certain thickness to restrict the height of a surface texture. In addition, etching thickness needs to be precisely controlled and re-produced during the etching process.
A further problem is that a variety of processes such as formation of a pattern for etching are required.